In a known apparatus described in the German Offenlegungsschrift No. 2 328 863, the local magnetic field is detected by a pair of perpendicular coils in which the varying magnetic field induces electrical voltages which become a measure of the distance of the coils and of the vehicle from the guide cable. These signals are processed in an electrical controller and finally become the error signal which is fed to a hydraulic final control element that causes the front wheels to be steered toward the cable so that the vehicle continues to track it. In the known apparatus, one coil is disposed vertically and the other horizontally and the voltages induced in the coils, when normalized, correspond to the following functions.
Vertical signal: Us=cos.alpha..multidot.sin.alpha..multidot.sin.omega.t and
Horizontal signal: Uw=cos.sup.2 .alpha..multidot.sin.omega.t
By forming the quotient Us/Us, one gets the linear function U=.sup.a/h wherein .alpha.=arc tan a/h, where h=constant, and the function U then becomes a measure of the deviation a of the vehicle from the ideal location defined by the guide cable.
It is a disadvantage of the known apparatus that the deviation of the vehicle from the proper track is represented as an analog signal and thus is not directly suitable for further processing in a digital computer circuit, for example in a microprocessor.